The term “sheet element processing machine” is here intended to comprise any machine which is being used for processing sheet elements such as paper, cardboard or similar materials, in particular printing machines, coating machines, laminating machines and converting machines (for example cutting, stamping, folding and/or gluing machines).
It is generally known to control the quality of sheet elements by means of a camera. Generally, the camera captures images of the sheet elements being transported through the quality control station. The captured images are analyzed with respect to many different parameters so as to obtain information whether or not the sheet elements fulfill certain criteria.
In order to allow the camera to capture the desired images, it is necessary to illuminate a viewing area of the camera which is the portion of the sheet element of which an image is being captured. It is important that the illumination unit provides light with an intensity which is as constant as possible.
While it is not too difficult to ensure that a certain, known viewing area is illuminated with a constant intensity, problems automatically arise if the quality control station is intended to receive sheet elements of different height e.g. thin paper and thick cardboard as this automatically result in a change of the “height” of the viewing area, the distance of the upper surface of the sheet element from a surface on which the sheet elements are being supported In a simple example, or assuming that cardboard with a thickness of 12 mm is being inspected, the viewing area is located 11.5 mm higher as compared to a situation where paper with a thickness of 0.5 mm is being inspected.